It is known to one skilled in the art that water-in-oil emulsions are difficult to stabilize against coalescence of the water droplets and therefore against the separating out of water. The reason behind this is due to the dense packing of the water droplets, as a consequence of the usually high content of disperse phase (>65%). A further consequence of the high packing density is a high viscosity per se of the emulsion. To achieve an adequate stability required by the cosmetic's industry, stabilizing waxes and, in some cases, high-viscosity oils, both of which increase the viscosity of the (external) oily phase, and thus reduce the mobility of the water droplets and protect against coalescence, are used. However, high-viscosity oils and stabilizing waxes have an adverse influence on the sensation on the skin, which manifests itself in a heavy and tacky feeling. Oil-in-water emulsions, which are known to have a lighter sensation on the skin, can in principle be used as an alternative, but water-in-oil emulsions have a significantly better care effect than oil-in-water emulsions.
A generally worthwhile aim is thus to make water-in-oil emulsions as light as possible, i.e., as low-viscosity as oil-in-water emulsions, without having to surrender their particular care effect. Furthermore, there is a need to make water-in-oil emulsions that are not tacky, yet are stable. The emulsifier to be used plays a particular key role in this context.
For ecological reasons, there is a considerable interest, both among producers and among consumers of emulsion preparations, in W/O emulsifiers which are based on natural raw materials. Partial esters of polyalcohols, such as glycerol, polyglycerol, sorbitol or methyl glycoside, and fatty acids, such as oleic or isostearic acid, find diverse uses for this reason.
However, this type of emulsifier is not suitable for stabilizng flowable emulsions (lotions) and creams with a high content of natural triglycerides. In addition to the W/O emulsifiers mentioned, the creams which meet the stability requirements of the market (resistance to temperatures from −15 to +45° C., in some cases from −25 to +50° C. comprise as lipid-like components predominantly paraffin oils and fatty acid ester of monoalkanols (MW<500); these have more favorable technological properties than the higher molecular weight triglycerides. Nevertheless, relatively high concentrations of viscosity-increasing waxes (≧3%) are required for the stabilization, these having an adverse effect on the application properties since they produce an undesirable tacky-greasy feeling on the skins.
Coemulsifiers, in particular ethylene oxide adducts in combination with metal soaps, extend the field of use only to lotions comprising paraffin oil.
The polyglycerol esters of dimeric and polymerized unsaturated C18-fatty acids have considerably better emulsifying properties than the polyalcohol fatty acid partial esters. They are obtained from the mono- and diglycerides of plant oils, preferably soya oil, by a heat treatment at approximately 300° C. for several hours or by transesterification of a thermally polymerized plant oil with polyglycerol.
The polyglycerol polyricinoleates formed from castor oil by an analogous process are also efficient W/O emulsifiers. This is described, for example, in DE-B44 09 569.
Both classes of substances have not been able to establish themselves in cosmetic or pharmaceutical emulsion preparations because of their sensitivity to oxidation and the often greasy-rancid smell. The massive exposure to heat during the preparation and the unsaturated character (iodine number approximately 100) are primarily responsible for this.
In contrast polyglycerol polyhydroxystearate, which is related chemically to polyglycerol polyricinoleate and can also be prepared from vegetable raw materials, has a satisfactory sensorial quality and has the main capability of forming cream-like and, in particular, flowable W/O emulsions.
DE-A40 12 693 has proposed esters of polycarboxylic acids with polyhydroxy compounds. Condensation of the starting compounds gives products with excess carboxyl groups, which are then neutralized. The reaction products are suitable as O/W emulsifiers.
EP-B-0 835 862 describes polyglyerol partial esters which are obtainable by esterification of a polyglycerol mixture having a degree of esterification of the polyglycerol of between 30 and 75% and saturated or unsaturated, linear or branched fatty acids having 12 to 22 C atoms and dimer fatty acids having an average functionality of 2 to 2.4. These have the additional advantage over polyglycerol polyhydroxystearate of an improved stability, in particular a higher resistance to freezing-thawing of the W/O emulsions prepared with them. However, the emulsions are still relatively viscous and cause a slightly tacky sensation on the skin.
The improvement in the resistance to freezing-thawing is of considerable practical interest for the transportability and storability of the emulsion preparations. By relatively long storage at very low temperatures or by extreme variations in temperature over relatively long transportation routes, the inadequate emulsion stabilization can manifest itself by significant separating out of water in the emulsion preparation or can even lead to complete breaking of the emulsion.